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designing a prescaler for a counter project - maybe ECL ?

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petrv

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I am designing a counter project using a PIC microcontroller and LCD display. 74LS or 74HC logic runs to about 30 MHz and 74AC to about 100 MHz (some manufacturers claim even 140-150 MHz) for their products. But what about higher frequencies ? There were some popular dividers/prescalers but they are all out of production (e.g. Philips/NXP SAB6456 div. 64 to 1.3 GHz or Fujitsu MB506 div. 128 working to 2.4 GHz. They both were very simple to use and worked well, but they are now obsolete ...). Looking at Mouser and DigiKey I have found only UPB1507 but datasheets says also minimal frequency 500 MHz :(
Then I had an idea to use ECL logic which is very fast - I have found IC's from On Semiconductor (avail. in Mouser) running up to 4 or even 4.5 GHz divider by 4 (MC100EL33 and MC100EP33). Unfortunately I have never tried to do anything with ECL, I wonder if anyone here has experience with ECL. I would like to chain 3 dividers by 4 to create ECL divider by 64 and then convert to conventional CMOS logic (74AC...). How should I terminate the differential ECL connection between those chips, proper PCB layout etc ...
I would like to use the ECL logic in PECL mode.

Thanks for any help.
Petr
 

crutschow

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Most Helpful Member
The MC100EP33 ECL circuit has differential inputs with a common-mode voltage of about 3.5V with a 5V supply. You can connect the differential inputs to the outputs directly when going ECL to ECL chip. For single ended input you tie one of the inputs to the VBB voltage (about 3.5V with a 5V supply). The other logic level input should then be driven between 3.1 and 4.0V.

You need an ECL to TTL converter to interface to CMOS logic. It converts the 3.1 to 4.0V ECL signal to the CMOS 0V to 5V levels.

Working at high frequencies requires transmission lines on all but the shortest connections (The maximum is approximately (3.5 * Tr)" for stripline where Tr is the signal risetime in ns. Thus for the 0.32ns risetime of the MC100EP33 the maximum unterminated trace length is 1.1".) PC traces must be laid out as microstrip or stripline to control the impedance. (A program to calculate trace impedances is available at UltraCAD Design, Ind.) The ends of the lines must be terminated in the characteristic impedance of the line with the resistor connected to a voltage equal to VBB. Typical PCB trace impedances are in the 100 ohm range but you need to calculate the value for the particular trace width, dielectric, etc. for the circuit board you are building.

Decoupling the IC is also critical. The circuit must be laid out on a PC board with a ground plane. A small (0.01uF) ceramic surface mount cap must be mounted as close as possible to each chip's power pin with the other end connected directly to ground plane. A good way to make this connection is to flood the top layer of the circuit board with ground plane between all traces and connect the capacitor directly to that. This flooded plane should then be connnect to an inner layer ground plane with numerous vias covering the board.

As is apparent, layout and decoupling are critical for high frequency circuits. Sloppy layout or interconnections will likely lead to the circuit not operating correctly.
 

kchriste

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Check out this page:
http://hem.passagen.se/communication/fcntlcd.html
He uses the LMX2322 as a prescaler in his counter circuit. You could probably adapt the newer LMX2326TM-ND from digikey to accomplish the same thing. You'd have to change both hardware and software to make it work. Pretty cheap at $3 and it goes up to 2.8 Ghz!.
 

petrv

New Member
Thanks to all of you who answered. As for the ECL: I intended just to chain 3 MC100EP33D, keep them as close as possible and the output connect to MC100ELT21D which will convert the signal to TTL as divided by 64 is already quite low frequency. So if I understand correctly I need to add 2 (small SMD) 50 ohm resistors to each pair and the other end connect to Vbb ? I'd like to avoid 4-layer PCB if possible (I am a hobbyist and not a professional and I use just EAGLE Standard, as I can't afford a more professional E-CAD software ...)
The samples are on the way from On Semiconductor to me ... :) so I will play with it a little.

As for the suggestion of using the LMX2326 PLL - this is an interesting one, I even have few of them in my drawer for a completely different project ....
If this would work that would be great, of course I know there is a prescaler inside that chip but I did not think I can get the output of the prescaler - the output used there is named "charge pump" and normally is connected to the loop filter and then controls the VCO tuning. I will test that, I am just waiting to receive my order of PCBs which include a small breakout board for that PLL
(which I intended to use in a different project :)

Petr
 

kchriste

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Forum Supporter
To use the prescaler on the LMX2326, set the FoLD MUX so that the Fo/LD (pin 14) output accesses the prescaler output. You'd do this by setting bits F3-F5 in the Function latch to 010.
 
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petrv

New Member
Thanks again, I will try that approach (as soon as I get a breakout board for TSSOP-16) I am trying to do something similar like the counter on the page you sent me, but more sophisticated, with more functions and options (including measuring low frequencies).
Petr
 

petrv

New Member
Thanks but I have already designed a breakout board specially designed for LMX2306/2316/2326 and submitted the design to batchpcb.com some time ago. Just received an e-mail that my PCBs were shipped to me, so now I am just waiting for the post...

I have designed a new breakout board for LMX234X - LMX2434 should go even to 5.0 GHz, it is TSSOP-20 package. I'll get soon the LMX2434 but until the new board is fabbed I'll try with LMX2326.
 
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